Substrate for information recording media and manufacturing method thereof, information recording medium, and starting material glass plate

ABSTRACT

A substrate for use as a disk substrate in a hard disk drive or the like, an information recording medium such as a magnetic disk, and a starting material glass plate which is a starting material of the substrate for information recording media. The forming conditions of the starting material glass plate are controlled such that the starting material glass plate has a long-wavelength waviness of not more than 6 nm. This starting material glass plate is polished so as to have a long-wavelength waviness of not more than 6 nm using CeO 2  abrasive grains having a mean grain diameter of not less than 0.0l μm and a 90% diameter of the volume grain size distribution of not less than 0.02 μm. The resulting substrate for an information recording medium has an excellent planarity, can be obtained in a short time and with a low polishing amount, and the resulting information recording medium is able to cope with increased data zone recording density.

This application is a Continuation Application under 35 USC 371 ofInternational Application PCT/JP01/02500 filed Mar. 27, 2001, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a substrate for information recordingmedia and a manufacturing method thereof, an information recordingmedium, and a starting material glass plate, and more specifically to asubstrate for information recording media for use as a disk substrate ina hard disk drive or the like and a manufacturing method thereof, aninformation recording medium such as a magnetic disk, an opticalmagnetic disk or an optical disk, and a starting material glass platefor use as a starting material of the substrate for informationrecording media.

BACKGROUND ART

In recent years, there has been remarkable progress in informationtechnology, and development of various types of information recordingmedium for storing information such as magnetic disks, optical magneticdisks and optical disks has been carried out with vigor.

Of these types of information recording medium, in the case for exampleof a magnetic disk, a magnetic film is formed on at least one surface ofa donut-shaped magnetic disk substrate, and recording and playback ofinformation are carried out by a magnetic head sliding over a data zonethus formed on the magnetic disk substrate.

As a method of manufacturing such a magnetic disk substrate, a methodhas been proposed in which a thin film of a magnetic material is formeddirectly on at least one surface of a sheet-shaped starting materialglass plate that has been manufactured by a float process or the like,without polishing the starting material glass plate (see, for example,Japanese Laid-open Utility Model Publication (Kokai) No. 60-159531).

However, with this manufacturing method, it is difficult to manufacturea magnetic disk having a good planarity sufficient to cope withincreases in data zone recording density that have occurred in recentyears. At present it is thus still common to polish the startingmaterial glass plate when manufacturing a magnetic disk substrate.

FIG. 1 is a flowchart showing a conventional method of manufacturingsuch a magnetic disk substrate.

In this conventional method, a starting material glass plate 101 is cutinto a donut shape in a disk processing step 102, then the inner andouter peripheral surfaces of the starting material glass plate 101 areprocessed to predetermined dimensions in an edge processing step 103,then the main surfaces of the starting material glass plate 101 arepolished in a surface polishing step 104, then the substrate isstrengthened if necessary in a chemical strengthening treatment step105, and then a finishing washing step 106 is carried out, thuscompleting the manufacture of the magnetic disk substrate 107.

In the surface polishing step 104, the polishing of the surfaces of thestarting material glass plate 101 is carried out in three stages, i.e.rough polishing 104 a, pre-polishing 104 b, and precision polishing 104c.

The surfaces of the magnetic disk substrate 107 manufactured from thesheet-shaped starting material glass plate 101 have an undulating shapeon a microscopic scale, in which a plurality of types of surfacewaviness classified by wavelength band are superimposed on one another,for example, long-wavelength waviness 108, medium-wavelength waviness109 and short-wavelength waviness 110, as shown in FIG. 2. A magnetichead 111 flies over the magnetic disk substrate 107 having this surfacewaviness.

However, as the data zone recording density has been increased in recentyears, the surface waviness properties have come to exert a largeinfluence on the electromagnetic conversion properties. If the abilityof the magnetic head to follow the surface waviness is poor, then therewill be a risk of malfunction during recording and/or playback, andhence a very high degree of planarity has come to be demanded ofmagnetic disk substrates.

The surface polishing step 104 is thus conventionally divided into threestages as described above. First, rough polishing 104 a using abrasivegrains having a relatively large mean diameter is carried out. As aresult, the thickness of the starting material glass plate 101 isadjusted to a predetermined value, and moreover surface waviness, inparticular long-wavelength waviness, is reduced, thus correcting thedegree of flatness of the starting material glass plate 101. Then, inthe pre-polishing 104 b and precision polishing 104 c, minute flawsformed on the surface of the starting material glass plate 101 andsurface waviness of relatively short wavelength (medium- andshort-wavelength waviness) are removed.

In recent years, to cope with increases in data zone recording density,there has been vigorous development of technology to reduce the size ofthe magnetic head 111, set the flying height of this small magnetic head111 to be low, and make the magnetic head 111 fly stably over themagnetic disk substrate 107 at this low flying height. Nowadays, thelength of magnetic heads has been reduced from about 2 mm to 1 mm orless.

As shown in FIG. 3, the long-wavelength waviness 108 is a relativelygently sloping waviness, and hence it is possible for the magnetic head111 to fly over the magnetic disk substrate 107 while following thelong-wavelength waviness 108 and thus maintaining a constant minute gapt between the magnetic head 111 and the magnetic disk substrate 107.

In contrast, as shown in FIG. 4, the medium-wavelength waviness 109 andthe short-wavelength waviness 110 have steep sloping parts 112, andhence the magnetic head 111 cannot fly over the magnetic disk substrate107 while maintaining a constant minute gap t between the magnetic head111 and the magnetic disk substrate 107 as is possible with thelong-wavelength waviness 108, i.e. the magnetic head 111 cannot followthe medium-wavelength waviness 109 and the short-wavelength waviness110. If medium-wavelength waviness 109 and short-wavelength waviness 110are present on the substrate surface, then this may thus causemalfunction during recording and/or playback. To obtain a magnetic disksubstrate of a desired high quality sufficient for coping with increaseddata zone recording density, it is thus necessary to carry out surfacepolishing so that medium-wavelength waviness 109 and short-wavelengthwaviness 110 are removed.

However, in the conventional manufacturing method described above,although in the rough polishing 104 a it is possible to correct thedegree of flatness, a lack of which is caused by long-wavelengthwaviness 108, new medium-wavelength waviness 109 and short-wavelengthwaviness 110 are formed on the surfaces of the starting material glassplate 101 through this rough polishing 104 a, and hence it is necessaryto increase the polishing amount in the pre-polishing 104 b and theprecision polishing 104 c. In the conventional manufacturing method,there is thus a problem that it is necessary to initially make thestarting material glass plate 101 thicker by a certain predeterminedamount, and moreover a large amount of polishing waste is dischargedduring the polishing, resulting in an increase in the amount ofindustrial waste, and an increase in the manufacturing cost.

Furthermore, the abrasive grains used in the rough polishing 104 a havea larger diameter than the abrasive grains used in the pre-polishing 104b and the precision polishing 104 c, and hence the surfaces of thestarting material glass plate 101 are easily scratched, and the amountof subsequent polishing must be increased to remove these surfacescratches; there is thus a problem that the starting material glassplate 101 must initially be made thicker by a certain predeterminedamount for this reason as well.

Moreover, because the rough polishing 104 a using abrasive grains havinga large diameter is carried out after the inner and outer peripheralsurfaces of the starting material glass plate 101 have been ground andpolished in the edge processing step 103, even if the inner and outerperipheral surfaces have been mirror-finished at considerable effort inthe edge processing step 103, they are polished once again with coarseabrasive grains during the rough polishing 104 a, and hence there is aproblem that the surface roughness of the inner and outer peripheralsurfaces drops, resulting in a drop in product quality.

Furthermore, because the surface polishing step 104 is carried out inthree divided stages (the rough polishing 104 a, the pre-polishing 104 band the precision polishing 104 c) as described above, there is aproblem that the number of steps required in the surface polishing ishigh and hence it takes a long time to complete the manufacturing, andmoreover there is a risk of the surfaces of the starting material glassplates 101 being scratched due to the substrates contacting one anotheror the jig or the like in each of the steps, and thus productivity ispoor.

DISCLOSURE OF THE INVENTION

In view of the problems described above, it is an object of the presentinvention to provide a substrate for information recording media havingsurfaces with a very high degree of planarity, and having a high productquality and a high reliability.

Moreover, it is another object of the present invention to provide amethod of manufacturing the above substrate for information recordingmedia, which enables the substrate for information recording media to bemanufactured easily in a short time and with a low polishing amount, andwhich thus enables productivity to be increased.

Furthermore, other objects of the present invention are to provide aninformation recording medium capable of coping with a high recordingdensity by using the above substrate for information recording media,and to provide a starting material glass plate suitable formanufacturing the substrate for information recording media.

The present invention will now be disclosed in outline form.

With information recording media such as magnetic disks nowadays,because magnetic heads have been reduced in size, it has become possiblefor a magnetic head to follow long-wavelength waviness, as described inthe section “Background Art” above (see FIG. 3). It is thus thought thatthe stage has been reached at which the necessity of the rough polishingstep in which medium-wavelength waviness and short-wavelength wavinessare newly produced should be investigated.

Regarding this point, it has already been proposed to manufacture aglass substrate for magnetic media (i.e. a substrate for informationrecording media) without carrying out lapping, which is rough polishing(Japanese Laid-open Patent Publication (Kokai) No. 2000-351653;hereinafter referred to as “the prior art”).

However, to cope with increases in data zone recording density, thesurfaces of the glass substrate must have an extremely high degree ofplanarity, and hence surface polishing must be carried out in some form.

However, in the above prior art, although the point “manufacture asubstrate for information recording media without carrying out roughpolishing (lapping)” is disclosed, there are no specific disclosureswhatsoever concerning what technical means can be used to secure a highdegree of planarity.

With regard to surface waviness formed on glass surfaces, defining thelong-wavelength waviness to be the average waviness Wa measured in awavelength band of 0.4 mm to 5.0 mm using an Optiflat optical-typesurface waviness measuring instrument made by Phase Shift Technology,the mediumwavelength waviness to be the average waviness Wa measured ina wavelength band of 0.4 mm to 2.0 mm using the above Optiflat measuringinstrument, and the short-wavelength waviness to be the averageroughness Ra measured in a wavelength band of 0.2 mm to 1.4 mm using aNewview 200 optical-type surface roughness meter made by Zygo, thepresent inventors thus carried out assiduous studies, and as a resultdiscovered that if the surface waviness of the starting material glassplate is good, then a substrate for information recording media havingan extremely good planarity can be obtained easily in a short time andwith a low polishing amount by carrying out only precision polishingusing ultra-fine abrasive grains of a certain predetermined diameter andwithout carrying out rough polishing.

Based on this discovery, a method of manufacturing a substrate forinformation recording media according to the present invention comprisesthe step of carrying out surface polishing on at least one surface of asheet-shaped starting material glass plate, wherein the surfacepolishing comprises only precision polishing using ultra-fine abrasivegrains having a first predetermined grain diameter.

According to the above method, the surface polishing is carried outusing only ultra-fine abrasive grains having a first predetermined graindiameter. As a result, there is no longer any new production ofmedium-wavelength waviness and/or short-wavelength waviness due to roughpolishing, and hence a high-quality substrate for information recordingmedia having an excellent planarity can be manufactured easily in ashort time and with a low polishing amount. Moreover, because thepolishing amount is low, the discharge of industrial waste such aspolishing waste can be suppressed, which is good environmentally.

Moreover, from the results of further experiments, the present inventorsdiscovered that by carrying out pre-polishing using fine abrasive grainshaving a second predetermined grain diameter larger than the firstpredetermined grain diameter before carrying out the precisionpolishing, a desired substrate for information recording media can beobtained in a yet shorter time without new medium-wavelength wavinessand/or short-wavelength waviness arising.

The method of manufacturing a substrate for information recording mediaaccording to the present invention thus alternatively comprises the stepof carrying out surface polishing on at least one surface of asheet-shaped starting material glass plate, wherein the surfacepolishing comprises only pre-polishing and precision polishing, whereinthe precision polishing is carried out using ultra-fine abrasive grainshaving a first predetermined grain diameter, and the pre-polishing iscarried out using fine abrasive grains having a second predeterminedgrain diameter larger than the first predetermined grain diameter.

Moreover, to avoid minute flaws from being formed on the surface(s) ofthe starting material glass plate, but also avoid dropping of thepolishing rate, the first predetermined grain diameter is preferablysuch that the mean grain diameter is in a range of 0.01 μm to 1.3 μm,and the 90% diameter of the volume grain size distribution (hereinafterreferred to merely as the “90% diameter”) is in a range of 0.02 μm to3.5 μm.

Furthermore, when the pre-polishing is carried out, from the viewpointof avoiding the formation of minute flaws and short-wavelength wavinesson the substrate for information recording media, it is preferable forthe second predetermined grain diameter to be such that the mean graindiameter is in a range of 0.3 μm to 5 μm, and the 90% diameter is in arange of 1 μm to 15 μm.

It should be noted that in the present invention, the “mean graindiameter” is the grain diameter at which the integrated volume grainsize becomes 50% of the entire integrated volume grainsize in the volumegrain size distribution when integration of the grain diameter iscarried out in order from the smallest grain diameter for the volumegrain size distribution, and the “90% diameter” is the grain diameter atwhich integrated volume grain size becomes 90% of the entire integratedvolume grain size in the volume grain size distribution when integrationof the grain diameter is carried out in order from the smallest graindiameter for the volume grain size distribution.

Moreover, the present inventors carried out further assiduous studies,and as a result discovered that, when manufacturing the sheet-shapedstarting material glass plate, by suitably controlling the formingconditions thereof, the long-wavelength waviness can be reliably keptdown to not more than 6 nm, and moreover that by carrying out surfacepolishing as described above on such a starting material glass platehaving a long-wavelength waviness of not more than 6 nm, a substrate forinformation recording media having extremely good surface wavinessproperties and an excellent planarity can easily be obtained.

In the method of manufacturing a substrate for information recordingmedia according to the present invention, it is thus preferable to use astarting material glass plate having a plurality of types of surfacewaviness classified by wavelength band formed in a fashion superimposedon one another on at least one surface of the starting material glassplate, wherein the long-wavelength waviness for which the wavelengthband is highest out of these types of surface waviness is formed to notmore than 6 nm.

Moreover, considering productivity, it is preferable for thelong-wavelength waviness of the starting material glass plate to beformed to not less than 0.4 nm. In addition, to carry out the precisionpolishing efficiently, it is preferable for the starting material glassplate to be formed such that the short-wavelength waviness for which thewavelength band is lowest is in a range of 0.1 nm to 0.7 nm, and themedium-wavelength waviness for which the wavelength band is intermediatebetween the wavelength band of the long-wavelength waviness and thewavelength band of the short-wavelength waviness is in a range of 0.25nm to 2 nm.

Furthermore, considering productivity and the like, it is preferable touse a float process as the method of manufacturing the starting materialglass plate having the long-wavelength waviness kept down to not morethan 6 nm. It is thus preferable to manufacture the starting materialglass plate from a glass ribbon in a predetermined high temperaturestate formed by pouring a glass raw material onto molten tin.

Moreover, to remove minute flaws from the at least one surface of thestarting material glass plate during the polishing while making thepolishing time as short as possible, it is preferable for the polishingamount in the surface polishing to be in a range of 1 μm to 75 μm, morepreferably 1 μm to 25 μm, from the at least one surface of the startingmaterial glass plate.

Furthermore, to carry out the precision polishing while maintaining agood polishing rate but without damaging the starting material glassplate, it is preferable to use at least one material selected from thegroup consisting of cerium oxide, aluminum oxide, zirconium oxide,silicon oxide and manganese oxide as the ultra-fine abrasive grains usedin the surface polishing; it is particularly preferable to use ceriumoxide.

Moreover, a substrate for information recording media according to thepresent invention is characterized by being manufactured by any of themanufacturing methods as described above, wherein a plurality of typesof surface waviness classified by wavelength band are formed in afashion superimposed on one another on at least one surface of thestarting material glass plate. Moreover, preferably, the long-wavelengthwaviness for which the wavelength band is highest is formed to 0.3 nm to1.2 nm, the short-wavelength waviness for which the wavelength band islowest is formed to 0.1 nm to 0.6 nm, and the medium-wavelength wavinessfor which the wavelength band is intermediate between the wavelengthband of the long-wavelength waviness and the wavelength band of theshort-wavelength waviness is formed to 0.2 nm to 0.9 nm.

According to the above constitution, a substrate for informationrecording media having good surface waviness properties and an excellentplanarity can be obtained easily and in a short time.

Moreover, an information recording medium according to the presentinvention comprises a substrate for information recording media asdescribed above, and an information recording layer formed on at leastone surface of the substrate.

According to the above constitution, an information recording mediumhaving an excellent planarity and thus capable of coping with increaseddata zone recording density can be obtained easily.

Moreover, a starting material glass plate according to the presentinvention, which has a plurality of types of surface waviness classifiedby wavelength band formed in a fashion superimposed on one another on atleast one surface thereof, is characterized in that the long-wavelengthwaviness for which the wavelength band is highest is formed to not morethan 6 nm. Moreover, preferably, the short-wavelength waviness for whichthe wavelength band is lowest is formed to not more than 0.7 nm, and themedium-wavelength waviness for which the wavelength band is intermediatebetween the wavelength band of the long-wavelength waviness and thewavelength band of the short-wavelength waviness is formed to not morethan 2 nm. Moreover, preferably, the starting material glass plate ismanufactured from a glass ribbon in a predetermined high temperaturestate formed by pouring a glass raw material onto molten tin.

According to the above constitution, a starting material glass platehaving the long-wavelength waviness suppressed can easily be obtained bya float process, and hence a starting material glass plate suitable formanufacturing a substrate for information recording media can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing a conventional method of manufacturing asubstrate for information recording media;

FIG. 2 is a schematic view showing a state of surface waviness on aconventional starting material glass plate;

FIG. 3 is a schematic view useful in explaining the relationship betweena magnetic head and a magnetic disk substrate in the case oflong-wavelength waviness;

FIG. 4 is a schematic view useful in explaining the relationship betweena magnetic head and a magnetic disk substrate in the case ofmedium-wavelength waviness or short-wavelength waviness;

FIG. 5 is a schematic sectional view showing principal parts of aninformation recording medium according to an embodiment of the presentinvention;

FIG. 6 is a flowchart showing a method of manufacturing a substrate forinformation recording media according to an embodiment of the presentinvention;

FIG. 7 is a schematic view showing the construction of a float plateglass manufacturing apparatus;

FIG. 8 is a flowchart showing a method of manufacturing a substrate forinformation recording media according to another embodiment of thepresent invention; and

FIG. 9 is a schematic view showing the construction of a down draw plateglass manufacturing apparatus.

BEST MODES FOR CARRYING OUT THE INVENTION

A detailed description will now be given of best modes for carrying outthe present invention, with reference to the drawings.

FIG. 5 is a schematic sectional view showing an information recordingmedium, specifically a magnetic disk, according to an embodiment of thepresent invention. The magnetic disk is comprised of a magnetic disksubstrate 1 having a foundation layer 2, a magnetic layer 3 and aprotective layer 4 formed in this order on a surface thereof using aknown sputtering method.

The magnetic disk substrate 1 is manufactured by a manufacturing methoddescribed below, and minute surface waviness is formed on the substratesurface. Specifically, the surface waviness is classified into the threetypes defined in the “Disclosure of the Invention” section above(long-wavelength waviness, medium-wavelength waviness andshort-wavelength waviness), and the magnetic disk substrate 1 is formedsuch that the long-wavelength waviness becomes 0.3 nm to 1.2 nm, themedium-wavelength waviness becomes 0.2 nm to 0.9 nm, and theshort-wavelength waviness becomes 0.1 nm to 0.6 nm.

The reasons for setting the long-wavelength waviness, themedium-wavelength waviness and the short-wavelength waviness of themagnetic disk substrate 1 to be in the above ranges will now be stated.

If the long-wavelength waviness exceeds 1.2 nm, and/or themedium-wavelength waviness exceeds 0.9 nm, and/or the short-wavelengthwaviness exceeds 0.6 nm, then the overall surface waviness will becomelarge, resulting in it being difficult for a magnetic head to follow thesurface waviness when flying at a low flying height over the magneticdisk substrate 1, which nowadays is made to have a high recordingdensity, and hence in it not being possible to obtain a high-qualitymagnetic disk substrate 1. On the other hand, even if thelong-wavelength waviness is made to be less than 0.3 nm, and/or themedium-wavelength waviness is made to be less than 0.2 nm, and/or theshort-wavelength waviness is made to be less than 0.1 nm, no furtherimprovement will be expected in the product quality, which will alreadyhave peaked. In the present embodiment, the magnetic disk substrate 1 isthus manufactured such that the long-wavelength waviness becomes 0.3 nmto 1.2 nm, the medium-wavelength waviness 0.2 nm to 0.9 nm, and theshort-wavelength waviness 0.1 nm to 0.6 nm.

It should be noted that because the three types of surface wavinessclassified by wavelength band are formed in a fashion superimposed onone another on the surface of the magnetic disk substrate 1, if even onetype of surface waviness out of the three types exceeds the range statedabove, then the electromagnetic conversion properties of the magnetichead will worsen, and hence the product quality of the magnetic disksubstrate 1 will drop. It is thus necessary for the ranges for all threetypes of surface waviness to be satisfied.

Moreover, in the magnetic disk, CrMo, Cr, CrV or the like can be used asthe material of the foundation layer 2, and a cobalt alloy such asCoPtCr or CoPtCrTa that enables excellent information recording/playbackcharacteristics and film adhesion to be secured can be used as thematerial of the magnetic layer 3. A carbon material such as hydrogenatedcarbon can be used as the material of the protective layer 4.

The method of manufacturing the magnetic disk substrate 1 will now bedescribed in detail.

FIG. 6 is a flowchart showing the method of manufacturing the magneticdisk substrate 1 according to a first embodiment of the presentinvention. Taking, for example, a piece of float plate glassmanufactured by a float process as the starting material glass plate 5,the magnetic disk substrate 1 is manufactured through a disk processingstep 6, an edge processing step 7, a surface polishing step 8, achemical strengthening treatment step 9 and a finishing washing step 10carried out in this order.

FIG. 7 is a schematic structural view showing a float plate glassmanufacturing apparatus. The float plate glass manufacturing apparatushas as principal parts thereof a melting furnace 11 into which apredetermined glass material powder is charged and in which the glassmaterial powder is melted under a predetermined high-temperatureatmosphere, a hermetically sealed forming bath 13 that is placed under areducing atmosphere and contains molten tin 12, and an annealing furnace15 in which a glass ribbon 14 drawn from the forming bath 13 isannealed.

There are no particular limitations on the glass materials. For example,a soda lime glass having SiO₂, Na₂O and CaO as principal componentsthereof, an aluminosilicate glass having SiO₂, Al₂O₃, Na₂O and Li₂O asprincipal components thereof, a borosilicate glass, an Li₂O-SiO₂ glass,an Li₂O—Al₂O₃—SiO₂ glass, or an RO—Al₂O₃—SiO₂ glass (wherein R=Mg, Ca,Sr or Ba) can be used. Moreover, a glass for glass strengthening inwhich ZrO₂, TiO₂ or the like has been added to glass materials as above,or a crystallized glass that is not subjected to chemical strengthening,can be used.

In the float plate glass manufacturing apparatus described above, theglass material powder, which has been prepared to a predeterminedcomposition, is charged into the melting furnace 11, which has beenheated to 1500 to 1600° C., whereupon the glass material powder ismelted in the melting furnace 11 to form a molten glass, which thenflows into the forming bath 13. As described above, the forming bath 13contains molten tin 12. The molten glass has a lower specific gravitythan the molten tin 12, and hence floats on top of the molten tin 12,moving in the direction of the arrow A in FIG. 7. As a result offloating on top of the molten tin 12, the molten glass is formed into aribbon shape, and hence becomes a glass ribbon 14 having a predeterminedthickness.

The glass ribbon 14 thus produced is drawn into the annealing furnace 15via a roller conveyor 16, and is conveyed in the direction of the arrowB shown in FIG. 7. The glass ribbon 14 is then cooled down to roomtemperature in the annealing furnace 15, this being done in such a wayas to prevent warping from occurring. The glass ribbon 14 that has beencooled to room temperature is then discharged from the annealing furnace15, and is cut into rectangular shapes, thus manufacturing a largenumber of starting material glass plates 5 from a single manufacturinglot.

In the float plate glass manufacturing apparatus described above, thelower surface of the glass ribbon 14 in contact with the molten tin 12is cooled while in contact with the molten tin 12, which has a freesurface, and hence the lower surface of the glass ribbon 14 is expectedto be extremely flat on a macroscopic scale. Moreover, as the glassribbon 14 is formed into a sheet shape of the predetermined thickness,the upper surface of the glass ribbon 14, which is in contact with aspace 17 above the molten tin 12, spreads out in a horizontal directionthrough viscous flow; the upper surface of the glass ribbon 14 shouldthus also be extremely flat on a macroscopic scale.

However, the molten tin 12 does not have a constant temperaturethroughout, but rather there is a temperature gradient in the directionof the arrow A shown in FIG. 7, and also the temperature is not uniformin the width direction of the glass ribbon 14 (the directionperpendicular to the plane of the paper in FIG. 7). Furthermore, thetemperature is also not constant throughout the space 17. As a result,even though the upper and lower surfaces of the glass ribbon 14 may haveexcellent planarity on a macroscopic scale, in general on a microscopicscale there will be quite a large surface waviness. Even if precisionpolishing described below is carried out on a starting material glassplate 5 having such a large surface waviness, then it will not bepossible to reduce the surface waviness to the desired value in a shorttime and with a low polishing amount. Moreover, if such a large surfacewaviness is to be removed, then it will be necessary to initially makethe starting material glass plate 5 extra thick.

In the present embodiment, by suitably controlling the temperatureinside the forming bath 13 and controlling the forming conditions, thestarting material glass plate 5 is thus formed in such a way that thesurface waviness of the starting material glass plate 5, in particularthe long-wavelength waviness, becomes small.

Specifically, by suitably controlling the temperature gradient in thedirection of the arrow A in FIG. 7 of the molten tin 12 in contact withthe glass ribbon 14, and carrying out temperature control such that thetemperature variation of the molten tin 12 in the width direction of theglass ribbon 14 is as small as possible, and moreover controllingconvection that occurs within the molten tin 12, and furthermorecontrolling the forming conditions such that the temperature variationand convection turbulence in the space 17 above the molten tin 12 becomesmall, a starting material glass plate 5 having the desired surfacewaviness properties can be manufactured. In particular, as the glassribbon 14 moves in the direction of the arrow A in FIG. 7, the viscosityof the glass increases, and hence to obtain the desired surface wavinessproperties, it is preferable for the temperature distribution in thewidth direction of the glass ribbon 14 to be made as uniform as possiblenear to the exit of the forming bath 13, and to stop small vibrationsfrom the outside from getting into the molten tin 12.

Specifically, it is preferable to form the starting material glass plate5 such that the long-wavelength waviness is 0.4 nm to 6 nm, themedium-wavelength waviness 0.25 nm to 2 nm, and the short-wavelengthwaviness 0.1 nm to 0.7 nm.

The reason for this is that if the long-wavelength waviness exceeds 6 nmand/or the medium-wavelength waviness exceeds 2 nm and/or theshort-wavelength waviness exceeds 0.7 nm, then to obtain a magnetic disksubstrate 1 having the desired good planarity, it will be necessary tomake the polishing amount large when carrying out the precisionpolishing in the surface polishing described below, and moreover thepolishing time will become long. On the other hand, it would bedifficult in terms of production technology to control the variousforming conditions such that the long-wavelength waviness is less than0.4 nm, the medium-wavelength waviness is less than 0.25 nm and theshort-wavelength waviness is less than 0.1 nm, and the production costwould rise.

In the present embodiment, the forming conditions are thus controlledwhen manufacturing the starting material glass plate 5 such that thelong-wavelength waviness becomes 0.4 nm to 6 nm, the medium-wavelengthwaviness 0.25 nm to 2 nm, and the short-wavelength waviness 0.1 nm to0.7 nm.

Subsequently, the starting material glass plate 5 having surfacewaviness properties as described above is subjected to various types ofprocessing, and the magnetic disk substrate 1 is manufactured as aproduct through the various steps mentioned above.

A description will now be given of these manufacturing steps in theorder in which they are carried out.

(1) Disk Processing Step 6

In the disk processing step 6, the starting material glass plate 5 iscut simultaneously along an outer peripheral surface and an innerperipheral surface thereof using a hard metal cutter or a diamondcutter, thus making the starting material glass plate 5 into a donutshape having predetermined outside and inside diameters and havingexcellent concentricity between the outer and inner peripheral surfaces.

It should be noted that, although in the present embodiment the outerperipheral surface and the inner peripheral surface are cutsimultaneously, it is also possible to first cut the outer peripheralsurface to the predetermined outside diameter and then bore a hole tothe predetermined inside diameter using a cylindrical diamondgrindstone, or to press the starting material glass plate 5 out into thepredetermined outside diameter and then bore a hole to the predeterminedinside diameter using such a diamond grindstone.

(2) Edge Processing Step 7

In the edge processing step 7, the outer and inner peripheral surfacesof the donut-shaped starting material glass plate 5 are ground andpolished so that the outside and inside diameters thereof become thedesired outside and inside diameters of the magnetic disk substrate 1,thus manufacturing a glass substrate. Specifically, grinding of theinner and outer peripheral surfaces is carried out in 2 stages usinggrindstones having diamond abrasive grains attached thereto, with thegrain size of the diamond abrasive grains being different in the twostages, and then chamfering of angular parts of the inner and outerperipheral surfaces is carried out.

The grain size of the diamond abrasive grains used is set as appropriateto be the most suitable in accordance with the required product quality.Moreover, it goes without saying that if, in the disk processing step 6described above, the starting material glass plate 5 has already beencut so as to have inside and outside diameters close to the desiredinside and outside diameters of the magnetic disk substrate 1, then itmay not be necessary to carry out the grinding divided into two stagesbut rather one stage may suffice.

The inner and outer peripheral surfaces (including the chamfered parts;likewise hereinafter) are then smoothed by polishing using CeO₂ (ceriumoxide) loose abrasive grains, so that the surface roughness Ra of theinner and outer peripheral surfaces becomes a predetermined value orless.

(3) Surface Polishing Step 8

In the surface polishing step 8, precision polishing 8 a is carried outon the main surfaces of the glass substrate while feeding onto thesesurfaces a polishing agent prepared by dispersing loose abrasive grainshaving a mean grain diameter in a range of 0.01 μm to 1.3 μm and a 90%diameter in a range of 0.02 μm to 3.5 μm (ultra-fine abrasive grains) ina polishing liquid.

The reasons for thus limiting the grain diameter of the loose abrasivegrains are as follows.

If the mean grain diameter exceeds 1.3 μm and/or the 90% diameterexceeds 3.5 μm, then the grain diameter of the loose abrasive grains asa whole will be large, and hence there will be a risk of newmedium-wavelength waviness and short-wavelength waviness being formedwhen the precision polishing is carried out, and moreover the surfacesof the glass substrate will become prone to being scratched by the looseabrasive grains. On the other hand, if the mean grain diameter is lessthan 0.01 μm and/or the 90% diameter is less than 0.02 μm, then thegrain diameter of the loose abrasive grains will be small, and hence thetime required for the polishing will become long, and thus theproductivity will drop.

In the present embodiment, the precision polishing 8 a is thus carriedout using loose abrasive grains having a mean grain diameter in a rangeof 0.01 μm to 1.3 μm and a 90% diameter in a range of 0.02 μm to 3.5 μm.

Moreover, in the precision polishing 8 a, the polishing amount is suchthat 1 μm to 75 μm, preferably 1 μm to 25 μm, of glass is removed fromeach of the surfaces of the glass substrate. Specifically, when carryingout precision polishing using loose abrasive grains having a graindiameter as described above on a glass substrate having a good surfacewaviness as described above, if the polishing amount is less than 1 μmthen, because the polishing amount is low, it will not be possible tosufficiently remove minute flaws formed on the surfaces of the glasssubstrate, whereas if the polishing amount exceeds 75 μm (preferably 25μm), then polishing will be carried out to an excessive extent, andhence time will be wasted on the polishing, resulting in a drop inproductivity.

In the present embodiment, the polishing amount in the precisionpolishing 8 a is thus made to be in a range of 1 μm to 75 μm, preferably1 μm to 25 μm, from the surfaces of the glass substrate.

There are no particular limitations on the type of the loose abrasivegrains; for example, it is possible to use a rare earth oxide such asCeO₂ or La₂O₃, or ZrO₂, MnO₂, Al₂O₃, SiO₂ (colloidal silica) or thelike. However, from the viewpoint of obtaining a good polishingefficiency, it is preferable to use rare earth oxide abrasive grains, inparticular CeO₂ abrasive grains.

Moreover, there are no particular limitations on polishing pads used inthe polishing; for example, nonwoven cloth polishing pads or foampolishing pads can be used. However, from the viewpoint of preventingthe formation of scratches on the glass substrate, it is preferable touse suede pads formed from a base layer and a NAP layer, where the NAPlayer is comprised of a continuous foam layer finished so as to formopenings in the surface thereof.

In the present embodiment, the precision polishing 8 a is carried out ata polishing rate in a range of 0.1 μm/min to 0.8 μm/min.

Moreover, after the precision polishing 8 a has been carried out, theglass substrate is washed using an acidic aqueous solution, an alkalineaqueous solution, pure water or the like.

(4) Chemical Strengthening Treatment Step 9

In the chemical strengthening treatment step 9, the glass substrate isimmersed for a predetermined time in a molten salt, for example a mixedmolten salt of potassium nitrate (KNO₃) and sodium nitrate (NaNO₃), thathas been adjusted to a predetermined temperature, thus carrying outchemical strengthening treatment in which ions such as Li⁺¹ and Na⁺¹ inthe chemical components of the glass substrate are exchanged with K₊₁,which has a higher ionic radius. By carrying out such chemicalstrengthening treatment, the surface compressive stress is raised, andhence the magnetic disk manufactured from the glass substrate can beprevented from breaking when rotated at high speed.

After the immersion in the molten salt, the glass substrate is annealeddown to a temperature close to room temperature, and then molten saltattached to the glass substrate is washed off in warm pure water.

It should be noted that, depending on the required strength of themagnetic disk substrate 1, this chemical strengthening treatment step 9can be omitted. Moreover, in the case that the starting material glassplate 5 is formed from a crystallized glass, chemical strengthening isnot possible, and hence the chemical strengthening treatment step 9 isnormally omitted.

(5) Finishing Washing Step 10

In the finishing washing step 10, the glass substrate that has beensubjected to the precision polishing and if necessary the chemicalstrengthening treatment is washed by immersing in an acidic aqueoussolution, an alkaline aqueous solution, pure water or a mixed solutionthereof and if necessary irradiating with ultrasound. Foreign mattersuch as polishing agent stuck to the surfaces of the glass substrate ormolten salt attached during the chemical strengthening treatment arethus removed, thus completing the manufacture of the magnetic disksubstrate 1.

As described above, in the present embodiment, in the surface polishingstep 8, only precision polishing 8 a is carried out using loose abrasivegrains (ultra-fine abrasive grains) having a mean grain diameter in arange of 0.01 μm to 1.3 μm and a 90% diameter in a range of 0.02 μm to3.5 μm (first predetermined grain diameter). The substrate forinformation recording media is thus manufactured without carrying outrough polishing, and hence there is no new formation ofmedium-wavelength waviness and/or short-wavelength waviness due to roughpolishing. Moreover, by selecting a starting material glass plate havinggood surface waviness properties, a high-quality substrate forinformation recording media having an excellent planarity can bemanufactured easily in a short time and with a low polishing amount.Moreover, because the polishing amount is low, the discharge ofindustrial waste such as polishing waste is suppressed, which isexcellent environmentally.

FIG. 8 is a flowchart showing a method of manufacturing a magnetic disksubstrate, i.e. a substrate for information recording media, accordingto a second embodiment of the present invention. In the presentembodiment, in a surface polishing step 8′, pre-polishing 8 b′ iscarried out before carrying out precision polishing 8 a′, thus furtherreducing the time required for the surface polishing step 8′.

The pre-polishing 8 b′ is carried out using loose abrasive grains (fineabrasive grains) having a grain diameter larger than the loose abrasivegrains used in the precision polishing 8 a′, specifically loose abrasivegrains having a mean grain diameter in a range of 0.3 μm to 5 μm and a90% diameter in a range of 1 μm to 15 μm.

The reasons for using loose abrasive grains having a grain diameter asabove in the pre-polishing 8 b′ are as follows.

If the mean grain diameter exceeds 5 μm and/or the 90% diameter exceeds15 μm, then the grain diameter will be large, resulting in the risk ofminute flaws occurring or short-wavelength waviness being formed. On theother hand, if the mean grain diameter is less than 0.3 μm and/or the90% diameter is less than 1 μm, then because the grain diameter issmall, it will not be possible to achieve the intended objective ofreducing the polishing time. In the present embodiment, thepre-polishing 8 b′ is thus carried out using loose abrasive grainshaving a mean grain diameter in a range of 0.3 μm to 5 μm and a 90%diameter in a range of 1 μm to 15 μm.

Moreover, the type of loose abrasive grains used in the pre-polishing 8b′ and the precision polishing 8 a′ may be the same as that used in theprecision polishing 8 a in the first embodiment, but it is alsopreferable for different types of loose abrasive grains to be usedbetween the pre-polishing 8 b′ and the precision polishing 8 a′. Forexample, the polishing can be carried out using a combination of Al₂O₃and CeO₂, CeO₂ and SiO₂ (colloidal silica), ZrO₂ and CeO₂, or CeO₂ andMnO₂ in the pre-polishing 8 b′ and the precision polishing 8 a′respectively.

In the second embodiment, the pre-polishing 8 b′ is thus carried outusing loose abrasive grains (fine abrasive grains) having a graindiameter (second predetermined grain diameter) larger than that of theloose abrasive grains used in the precision polishing 8 a′, specificallya mean grain diameter in a range of 0.3 μm to 5 μm and a 90% diameter ina range of 1 μm to 15 μm, and then the precision polishing 8 a′ iscarried out. As a result, the polishing time required for the surfacepolishing can be further reduced, and hence the productivity ofmanufacturing a magnetic disk substrate 1 having a high product qualityand an excellent reliability can be improved.

FIG. 9 is a schematic structural view showing a down draw plate glassmanufacturing apparatus, which is another embodiment of the apparatusfor manufacturing the starting material glass plate. The down draw plateglass manufacturing apparatus has as principal parts thereof a meltingfurnace 21 into which a predetermined glass material powder is chargedand in which the glass material powder is melted under a predeterminedhigh-temperature atmosphere, a working bath 22 in which the molten glassis adjusted to a predetermined temperature, a slot 23 that has arestriction and is formed from platinum and through which the moltenglass is drawn from the working bath 22 to form a glass ribbon 25, andan annealing furnace 24 in which the glass ribbon 25 is annealed.

In the down draw plate glass manufacturing apparatus constructed asabove, the glass material powder, which has been prepared to apredetermined composition, is charged into the melting furnace 21, whichhas been heated to 1500 to 1600° C., whereupon the glass material powderis melted in the melting furnace 21 to form the molten glass. The moltenglass then flows into the working bath 22, where the molten glass ishomogenized and adjusted to a temperature suitable for the forming. Themolten glass then flows out downwards from the working bath 22 via theslot 23. The speed of dropping of the molten glass is adjusted to apredetermined speed by means of gravity (shown by the arrow C in FIG. 9)and the rotational force from rollers 26, thus forming a glass ribbon 25having a predetermined thickness. The glass ribbon 25 is then cut intopredetermined rectangular shapes, thus obtaining starting material glassplates.

In this down draw method, the temperature of the molten glass passingthrough the slot 23 and the temperature of the slot 23 itself, and thetemperature distribution in the width direction of the slot 23,determine the thickness distribution and the surface quality such as theplanarity (waviness properties) of the glass ribbon 25 (the startingmaterial glass plates). Conditions are thus devised such that a glassribbon 25 having excellent product quality can be obtained, for examplea platinum restriction can be used as the slot 23 as shown in FIG. 9, oralternatively a refractory fusion pipe can be used.

Moreover, in the down draw method, the glass ribbon 25 drawn from theslot 23 drops through free space inside the annealing furnace 24 undergravity while an end part thereof is held between the rollers 26. Thespeed of dropping and temperature gradient of the molten glass, and theair flow and temperature distribution in the width direction inside thefree space through which the molten glass passes are controlled suchthat the surface waviness of the glass ribbon 25 (the starting materialglass plates) is low. As a result, a glass ribbon 25 (starting materialglass plates) having a long-wavelength waviness in a range of 0.4 nm to6 nm, a medium-wavelength waviness in a range of 0.25 nm to 2 nm, and ashort-wavelength waviness in a range of 0.1 nm to 0.7 nm ismanufactured.

It should be noted that the present invention is not limited to theembodiments described above. For example, in the embodiments describedabove the starting material glass plate 5 was manufactured using a floatmethod or a down draw method, but so long as a starting material glassplate 5 having a long-wavelength waviness in a range of 0.4 nm to 6 nm,a medium-wavelength waviness in a range of 0.25 nm to 2 nm, and ashort-wavelength waviness in a range of 0.1 nm to 0.7 nm can bemanufactured, there are no particular limitations on the manufacturingmethod. For example, the starting material glass plate 5 may bemanufactured using a redraw method in which a mother glass sheet thathas been formed into a sheet shape using a melting bath or the like isreheated in a heating furnace to lower the viscosity of the glass, themother glass sheet is made thinner by drawing in a downwards directionor a horizontal direction, and then annealing is carried out in anannealing furnace.

EXAMPLES

A description will now be given of specific examples of the presentinvention.

First Examples

The present inventors manufactured the following test pieces: usingpieces of float plate glass (starting material glass plates) having goodsurface waviness properties, test pieces for which only precisionpolishing was carried out (Examples 1 to 5), and test pieces for whichpre-polishing and then precision polishing were carried out (Examples 6to 8); using pieces of down draw plate glass (starting material glassplates) having good surface waviness properties, test pieces for whichonly precision polishing was carried out (Example 9); using float plateglass (starting material glass plates) having good surface wavinessproperties, test pieces for which rough polishing was incorporated intothe surface polishing step (Comparative Examples 1 and 2); and usingstarting material glass plates having surface waviness outside theranges according to the present invention, test pieces for which varioustypes of surface polishing were carried out (Comparative Examples 3 to5). For each of the test pieces, the surface waviness was measured andthus the surface properties evaluated before and after the surfacepolishing, i.e. immediately after the grinding of the inner and outerperipheral surfaces and then after the precision polishing had beencompleted.

For each of the examples and comparative examples, Table 1 shows themethod of manufacturing the starting material glass plates, the surfaceproperties of the starting material glass plates, and the details of thesurface polishing; Table 2 shows, amongst other things, the surfaceproperties of the magnetic disk substrates obtained.

It should be noted that the long-wavelength waviness and themedium-wavelength waviness were measured over a diameter range of 38 mmto 84 mm using an Optiflat optical-type surface waviness measuringinstrument made by Phase Shift Technology, and the short-wavelengthwaviness was measured at three places, namely the inner periphery, theouter periphery and a circle midway between the inner periphery and theouter periphery, using a Newview 200 optical-type surface roughnessmeter made by Zygo, and mean values were calculated. Moreover, themeasurement values shown in the tables are mean values over a largenumber of test pieces (300 or 500) manufactured in the example orcomparative example in question.

Moreover, the polishing amount was calculated by measuring the thicknessof each test piece before and after the polishing using a micrometermade by Mitsutoyo Corporation.

TABLE 1 Starting Material Glass Plate Surface Polishing Surface WavinessRough Polishing Pre-Polishing (nm) Mean Mean Long Medium Short Grain 90%Polishing Grains 90% Manufacturing Wave- Wave- Wave- Abrasive DiameterDiameter Time Abrasive Diameter Diameter Method length length lengthGrains (μm) (μm) (min) Grains (μm) (μm) Examples 1 Float Method 3.5 1.60.7 — — — — — — — 2 Float Method 3.5 1.6 0.7 — — — — — — — 3 FloatMethod 3.6 1.2 0.4 — — — — — — — 4 Float Method 3.6 1.2 0.4 — — — — — —— 5 Float Method 1.2 0.6 0.3 — — — — — — — 6 Float Method 3.5 1.6 0.7 —— — — CeO₂ 3 8   7 Float Method 3.6 1.2 0.4 — — — — CeO₂ 3 7.5 8 FloatMethod 3.6 1.2 0.4 — — — — CeO₂ 1 3   9 Down Draw 1.8 0.9 0.4 — — — — —— — Method Comparative 1 Float Method 3.5 1.6 0.7 Al₂O₃ 5.5 10 35 — — —Examples 2 Float Method 3.5 1.6 0.7 Al₂O₃ 9   20 20 CeO₂ 3 7.5 3 FloatMethod 7.0 2.6 1.0 Al₂O₃ 9   20 20 — — — 4 Float Method 7.0 2.6 1.0 — —— — CeO₂ 3 8   5 Float Method 7.0 2.6 1.0 — — — — — — — SurfacePolishing Pre-Polishing Precision Polishing Mean Total Polishing Grain90% Polishing Polishing Time Abrasive Diameter Diameter Time Time (min)Grains (μm) (μm) (min) (min) Examples 1 — CeO₂ 1 3 40 40 2 — CeO₂ 1 3 6060 3 — CeO₂ 1 2.8 60 60 4 — CeO₂ 0.3 1.2 120 120 5 — CeO₂ 1 3 20 20 6  7CeO₂ 1 3 20 27 7  9 CeO₂ 0.3 1.2 30 39 8 40 Colloidal Silica 0.02 0.0330 70 9 — CeO₂ 1 3 40 40 Comparative 1 — CeO₂ 1 3 40 75 Examples 2 30CeO₂ 1 3 40 90 3 — CeO₂ 1 3 35 55 4 60 CeO₂ 1 3 40 100 5 — CeO₂ 1 3 200200

TABLE 2 Number of Magnetic Disk Substrate Polishing Surface SurfaceWaviness (nm) Amount Polishing Long Medium Short Minute (μm) StepsWavelength Wavelength Wavelength Flaws Evaluation Examples 1 10 1 1.00.6 0.4 1/300 ◯ 2 15 1 0.8 0.5 0.3 2/300 ◯ 3 15 1 0.8 0.6 0.3 2/500 ◯ 412 1 1.1 0.6 0.15 1/300 ◯ 5 5 1 0.6 0.4 0.2 1/300 ◯ 6 10 2 1.0 0.65 0.44/300 ◯ 7 10 2 0.5 0.35 0.2 5/500 ◯ 8 10.3 2 0.9 0.6 0.2 1/500 ◯ 9 10 10.5 0.3 0.2 6/300 ◯ Comparative 1 110 2 1.0 0.8 1.5 72/300  X Examples 2230 3 0.8 0.5 0.35 15/300  Δ 3 210 2 1.7 1.0 1.8 105/300  X 4 50 2 1.10.7 0.5 3/300 Δ 5 50 1 1.2 0.7 0.45 4/300 Δ

A description will now be given of the procedure for manufacturing thetest pieces for each of the Examples and Comparative Examples.

Example 1

The present inventors first manufactured lithium-alumina-silica typestarting material glass plates using a float plate glass manufacturingapparatus as shown in FIG. 7. Specifically, glass material powders werecharged into the melting furnace such that the glass composition wouldbe 70 mol % SiO₂, 15 mol % Al₂O₃, 7 mol % Li₂O and 8 mol % Na₂O, andwere melted in the melting furnace. The molten mixture was then made toflow into the forming bath, thus producing a glass ribbon, next theglass ribbon was moved along the top of the molten tin while controllingthe temperature in the forming bath according to predetermined formingconditions enabling good surface waviness properties to be obtained, andthen the glass ribbon was conveyed from the forming bath into theannealing furnace. The glass ribbon thus obtained from the manufacturinglot was then cut into rectangles, thus obtaining 300 starting materialglass plates of thickness approximately 1 mm.

Next, each starting material glass plate was processed into a donutshape by cutting simultaneously along the outer peripheral surface andthe inner peripheral surface thereof using a diamond cutter, such thatthe outside diameter became 95 mm and the inside diameter 25 mm.

After that, grinding of the inner and outer peripheral surfaces andchamfering of angular parts thereof were carried out using grindstoneshaving diamond abrasive grains attached thereto, and then the inner andouter peripheral surfaces including the chamfered parts weremirror-finished by polishing using CeO₂ abrasive grains.

Next, precision polishing of the surfaces of each starting materialglass plate was carried out for 40 minutes while feeding onto thesurfaces of the starting material glass plate a polishing agent preparedby dispersing CeO₂ abrasive grains having a mean grain diameter of 1 μmand a 90% diameter of 3 μm in a polishing liquid, thus preparing thetest pieces of Example 1. Note that suede pads were used as thepolishing pads.

Example 2

300 starting material glass plates from the same manufacturing lot as inExample 1 were subjected to the same disk processing and edge processingas in Example 1, and then precision polishing was carried out for 60minutes using the same CeO₂ abrasive grains as in Example 1, thuspreparing the test pieces of Example 2.

Example 3

Another manufacturing lot of glass ribbon was prepared using the sameglass material powder composition and forming conditions as inExample 1. The glass ribbon was then cut into rectangles, thus obtaining500 starting material glass plates of thickness approximately 1 mm.

Next, the same disk processing and edge processing as in Example 1 werecarried out, and then precision polishing was carried out for 60 minutesusing CeO₂ abrasive grains having a mean grain diameter of 1 μm and a90% diameter of 2.8 μm, thus preparing the test pieces of Example 3.

Example 4

300 starting material glass plates were obtained from the samemanufacturing lot as in Example 3 and subjected to the same diskprocessing and edge processing as in Example 1, and then precisionpolishing was carried out for 120 minutes using CeO₂ abrasive grainshaving a mean grain diameter of 0.3 μm and a 90% diameter of 1.2 μm,thus preparing the test pieces of Example 4.

Example 5

A glass ribbon was prepared using the same glass material powdercomposition as in Example 1, but changing the forming conditions so asto make the surface waviness yet smaller. The glass ribbon was then cutinto rectangles, thus obtaining 300 starting material glass plates ofthickness approximately 1 mm.

Next, the same disk processing and edge processing as in Example 1 werecarried out, and then precision polishing was carried out for 20 minutesusing the same CeO₂ abrasive grains as in Example 1, thus preparing thetest pieces of Example 5.

Example 6

300 starting material glass plates were obtained from the samemanufacturing lot as in Example 1 and subjected to the same diskprocessing and edge processing as in Example 1, then pre-polishing wascarried out for 7 minutes using CeO₂ abrasive grains having a mean graindiameter of 3 μm and a 90% diameter of 8 μm, and then precisionpolishing was carried out for 20 minutes using the same CeO₂ abrasivegrains as in Example 1 (mean grain diameter 1 μm, 90% diameter 3 μm),thus preparing the test pieces of Example 6. In Example 6, 7 minutes ofpre-polishing and 20 minutes of precision polishing, i.e. a total of 27minutes of surface polishing, were thus carried out.

Example 7

500 starting material glass plates were obtained from the samemanufacturing lot as in Example 3 and subjected to the same diskprocessing and edge processing as in Example 1, then pre-polishing wascarried out for 9 minutes using CeO₂ abrasive grains having a mean graindiameter of 3 μm and a 90% diameter of 7.5 μm, and then precisionpolishing was carried out for 30 minutes using the same CeO₂ abrasivegrains as in Example 4 (mean grain diameter 0.3 μm, 90% diameter 1.2μm), thus preparing the test pieces of Example 7. In Example 7, 9minutes of pre-polishing and 30 minutes of precision polishing, i.e. atotal of 39 minutes of surface polishing, were thus carried out.

Example 8

500 starting material glass plates were obtained from the samemanufacturing lot as in Example 3 and subjected to the same diskprocessing and edge processing as in Example 1, then pre-polishing wascarried out for 40 minutes using CeO₂ abrasive grains having a meangrain diameter of 1 μm and a 90% diameter of 3 μm, and then precisionpolishing was carried out for 30 minutes using colloidal silica abrasivegrains having a mean grain diameter of 0.02 μm and a 90% diameter of0.03 μm, thus preparing the test pieces of Example 8. In Example 8, 40minutes of pre-polishing and 30 minutes of precision polishing, i.e. atotal of 70 minutes of surface polishing, were thus carried out.

Example 9

Starting material glass plates were manufactured using the same glassmaterial powder composition as in Example 1, but using a down draw plateglass manufacturing apparatus as shown in FIG. 9. Specifically, theglass material powders were charged into the melting furnace to preparea molten glass, then the molten glass was made to flow out downwardsfrom the platinum orifice (slot) using gravity to form a glass sheet,then annealing was carried out in the annealing furnace, and then theglass sheet was cut into rectangles, thus obtaining 300 startingmaterial glass plates.

Disk processing, edge processing and precision polishing were thencarried out as in Example 1, thus preparing the test pieces of Example9.

Comparative Example 1

300 starting material glass plates were obtained from the samemanufacturing lot as in Example 1 and subjected to the same diskprocessing and edge processing as in Example 1, then rough polishing wascarried out for 35 minutes using Al₂O₃ abrasive grains having a meangrain diameter of 5.5 μm and a 90% diameter of 10 μm, and then precisionpolishing was carried out as in Example 1, thus preparing the testpieces of Comparative Example 1. In Comparative Example 1, 35 minutes ofrough polishing and 40 minutes of precision polishing, i.e. a total of75 minutes of surface polishing, were thus carried out.

Comparative Example 2

300 starting material glass plates were obtained from the samemanufacturing lot as in Example 1 and subjected to the same diskprocessing and edge processing as in Example 1, and then surfacepolishing divided into the three stages of rough polishing,pre-polishing and precision polishing as conventionally was carried out.Specifically, rough polishing was first carried out for 20 minutes usingAl₂O₃ abrasive grains having a mean grain diameter of 9 μm and a 90%diameter of 20 μm, then pre-polishing was carried out for 30 minutesusing CeO₂ abrasive grains having a mean grain diameter of 3 μm and a90% diameter of 7.5 μm, and then precision polishing was carried out for40 minutes using the same CeO₂ abrasive grains as in Example 1 (meangrain diameter 1 μm, 90% diameter 3 μm), thus preparing the test piecesof Example 2. In Comparative Example 2, 20 minutes of rough polishing,30 minutes of pre-polishing and 40 minutes of precision polishing, i.e.a total of 90 minutes of surface polishing, were thus carried out.

Comparative Example 3

Using the same glass material powder composition as in Example 1, aglass ribbon was manufactured using a float plate glass manufacturingapparatus but without carrying out sufficient temperature control in theforming bath, and then the glass ribbon was cut into rectangles, thusobtaining 300 starting material glass plates of thickness approximately1mm.

Next, the same disk processing and edge processing were carried out asin Example 1, then rough polishing was carried out for 20 minutes as inComparative Example 2, and then precision polishing was carried out for35 minutes as in Example 1, thus preparing the test pieces ofComparative Example 3. In Comparative Example 3, 20 minutes of roughpolishing and 35 minutes of precision polishing, i.e. a total of 55minutes of surface polishing, were thus carried out.

Comparative Example 4

300 starting material glass plates were obtained from the samemanufacturing lot as in Comparative Example 3 and subjected to the samedisk processing and edge processing as in Example 1, and then 60 minutesof pre-polishing and 40 minutes of precision polishing were carried outunder the same conditions as in Example 6, thus preparing the testpieces of Comparative Example 4.

Comparative Example 5

300 starting material glass plates were obtained from the samemanufacturing lot as in Comparative Example 3 and subjected to the samedisk processing and edge processing as in Example 1, and then 200minutes of precision polishing was carried out using the same CeO₂abrasive grains as in Example 1, thus preparing the test pieces ofComparative Example 5.

As can be seen from the measurement results in Table 1 and Table 2, thelong-wavelength waviness, the medium-wavelength waviness and theshort-wavelength waviness of the starting material glass plates inExample 1 were all low at 3.5 nm, 1.6 nm and 0.7 nm respectively. Afterthe precision polishing, the long-wavelength waviness, themedium-wavelength waviness and the short-wavelength waviness of themagnetic disk substrates were thus also all low at 1.0 nm, 0.6 nm and0.4 nm respectively. It was thus possible to manufacture magnetic disksubstrates having an excellent planarity, with the medium-wavelengthwaviness and the short-wavelength waviness in particular beingsuppressed. Furthermore, the polishing amount was low at 10 μm, andhence it was possible to obtain excellent surface waviness propertieswith a low polishing amount. Moreover, when it was verified by visualobservation whether or not minute flaws had occurred, a good result wasobtained, namely there were minute flaws on only 1 out of the 300 testpieces. It was thus found that the probability of minute flaws occurringis extremely low.

In Example 2, the polishing time was increased by 1.5 times comparedwith Example 1. Because the polishing time was increased, the polishingamount became somewhat higher at 15 μm, but the surface wavinessproperties of the magnetic disk substrates were further improved.

In Example 3, the magnetic disk substrates were prepared from startingmaterial glass plates under approximately the same conditions as inExample 2. It can be seen from the tables that similar surface wavinessproperties to Example 2 were thus obtained.

In Example 4, the grain diameter of the CeO₂ abrasive grains used as theloose abrasive grains in the precision polishing was reduced comparedwith Examples 1 to 3, specifically, the mean grain diameter was made tobe 0.3 μm and the 90% diameter 1.2 μm. The polishing rate was thussomewhat slower, and hence the polishing amount was low at 12 μm despitea long polishing time of 120 minutes. However, the short-wavelengthwaviness was improved compared with Examples 1 to 3. It was thus foundthat the short-wavelength waviness can be reduced by carrying out theprecision polishing using finer loose abrasive grains.

In Example 5, the surface waviness properties of the starting materialglass plates were yet better than in Examples 1 to 4, in particular thelong-wavelength waviness. It was thus possible to manufacture magneticdisk substrates having an extremely good planarity in a short polishingtime of 20 minutes and with a low polishing amount of 5 μm.

In Example 6, pre-polishing was carried out using CeO₂ abrasive grainshaving a mean grain diameter of 3 μm and a 90% diameter of 8 μm, andthen precision polishing was carried out as in Example 1. Because thepre-polishing was carried out, it was possible to obtain magnetic disksubstrates having excellent surface waviness properties in a shortertime than in Example 1. However, because the prepolishing was carriedout using CeO₂ abrasive grains having a larger grain diameter than CeO₂abrasive grains used in the precision polishing, the frequency ofoccurrence of minute flaws was somewhat greater than when only precisionpolishing was carried out, at 4 out of 300 test pieces. Nevertheless,the yield was 99%, and hence sufficiently satisfactory results wereobtained.

As can be seen from Examples 1 to 6, given that the starting materialglass plates have approximately the same surface waviness properties,the total polishing time can be reduced by carrying out pre-polishing.

In Example 7, pre-polishing was also carried out using CeO₂ abrasivegrains approximately the same as those used in Example 6, and thenprecision polishing was carried out. However, the CeO₂ abrasive grainsused in the precision polishing had a low mean grain diameter and 90%diameter of 0.3 μm and 1.2 μm respectively. It can be seen that as aresult it was possible to obtain magnetic disk substrates havingextremely good surface waviness properties in a short time. Moreover,although the frequency of occurrence of minute flaws was somewhatincreased compared with when only precision polishing was carried out at5 out of 500 magnetic disk substrates, the yield was nevertheless 99%,and hence sufficiently satisfactory results were obtained.

In Example 8, pre-polishing was carried out using CeO₂ abrasive grainshaving a low grain diameter overall, specifically a 90% diameter of 3μm, and then precision polishing was carried out using fine colloidalsilica. Because the grain diameter was low, the polishing rate wassomewhat slow, specifically the polishing amount was 10.3 μm with apolishing time of 70 minutes. However, good surface waviness propertieswere obtained.

In Example 9, the starting material glass plates were manufactured usingthe down draw method. It was found that even in the case ofmanufacturing the starting material glass plates using the down drawmethod, by suitably carrying out temperature control, starting materialglass plates having excellent surface waviness properties can beprepared, and moreover that so long as the starting material glassplates have good surface waviness properties, magnetic disk substrateshaving an excellent planarity (surface waviness properties) can beobtained regardless of the method of manufacturing the starting materialglass plates.

As described above, in Examples 1 to 9, by using starting material glassplates having excellent surface waviness properties, it was possible tomanufacture magnetic disk substrates having an excellent planarity withdesired surface waviness properties by carrying out only precisionpolishing or only pre-polishing and precision polishing with no roughpolishing. Moreover, because rough polishing was not carried out, it waspossible to manufacture the magnetic disk substrates in a short time andwith a low polishing amount. Furthermore, it was verified that theprobability of occurrence of minute flaws was extremely low at 1% orless.

In contrast with the above, in Comparative Example 1, rough polishingusing Al₂O₃ abrasive grains having a mean grain diameter of 5.5 μm and a90% diameter of 10 μm was carried out on starting material glass platesobtained from the same manufacturing lot as in Example 1. The surfacewaviness properties of the magnetic disk substrates measured after theprecision polishing were that the long-wavelength waviness and themedium-wavelength waviness were low at 1.0 nm and 0.8 nm respectively,but the short-wavelength waviness had increased to 1.5 nm. It is thoughtthat this is because short-wavelength waviness was newly formed throughcarrying out the rough polishing. Incidentally, the surface wavinessproperties were also measured immediately after the rough polishing,whereupon it was found that the long-wavelength waviness had beensuppressed to 2.5 nm, but the medium-wavelength waviness and theshort-wavelength waviness were large at 1.8 nm and 2.2 nm respectively.Moreover, it was found through visual inspection that there were minuteflaws on 72 out of the 300 test pieces, and hence it was found thatdefective articles were manufactured at a probability of over 20%.

In Comparative Example 2, rough polishing similar to that of ComparativeExample 1 was carried out on starting material glass plates obtainedfrom the same manufacturing lot as in Example 1. It is thus assumed thatmedium-wavelength waviness and short-wavelength waviness were newlyformed through the rough polishing. Although it was possible to obtaingood magnetic disk substrate surface waviness properties as shown inTable 2, to achieve this it was necessary to carry out a large amount ofpolishing, specifically 230 μm, on the substrate surfaces. In such acase, it is necessary to initially make the starting material glassplates thicker, and moreover a large amount of polishing waste isdischarged, resulting in an increase in the amount of industrial waste.

In Comparative Example 3, rough polishing and precision polishing werecarried out as in Comparative Example 1, but using starting materialglass plates having poor surface waviness properties. Even though alarge amount of polishing, specifically 210 μm, was carried out, thelong-wavelength waviness, the medium-wavelength waviness and theshort-wavelength waviness of the magnetic disk substrates were thus allpoor at 1.7 nm, 1.0 nm and 1.8 nm respectively. Moreover, it was foundthrough visual inspection that there were minute flaws on 105 out of 300of the test pieces, and hence the proportion of test pieces on whichminute flaws occurred was greater than 30%.

In Comparative Example 4, pre-polishing and precision polishing werecarried out as in Example 6 but using starting material glass platesobtained from the same manufacturing lot as in Comparative Example 3. Itwas possible to obtain magnetic disk substrates having good surfacewaviness properties as shown in Table 2, but because the surfacewaviness properties of the starting material glass plates were poor, toobtain magnetic disk substrates having good surface waviness properties,it was necessary to make the polishing amount high at 50 μm, andmoreover the total polishing time was long at 100 minutes; thismanufacturing method is thus unsuited to mass production.

In Comparative Example 5, starting material glass plates from the samemanufacturing lot as in Comparative Example 3 were used, and moreoveronly precision polishing as in Example 1 was carried out. As a result,to obtain good surface waviness properties, a very long polishing timeof 200 minutes was required; this manufacturing method is thus veryunsuited to mass production.

Second Examples

Next, using test pieces from Example 1 and Comparative Example 1, thepresent inventors used a known sputtering method to build up afoundation layer made of CrMo, a magnetic layer made of CoCrPt and aprotective layer made of hydrogenated carbon in this order on thesurfaces of each test piece, thus preparing magnetic disks. Touch downheight tests were then carried out, and the modulation was measured.

[Touch Down Height Tests]

The present inventors carried out touch down height tests in which amagnetic head is lowered down while rotating a magnetic disk, and theminimum flying height at which the magnetic head can fly stably isevaluated. For the test pieces from Comparative Example 1, the touchdown height was high at 11 nm, and there was thus a risk that it wouldnot be possible to cope with a low flying height of 10 nm or less. Withthe test pieces from Example 1, on the other hand, a good result of thetouch down height being 5 nm or less was obtained, and hence it wasverified that the magnetic disks were suitable for use at a low flyingheight.

[Modulation Measurements]

The modulation M is defined as in equation (1) below, where Vmax (mV)and Vmin (mV) are the maximum output and minimum output of the magneticdisk as measured using an oscilloscope. It is considered that for amagnetic disk to have a good flatness with little surface undulation, itis preferable for the modulation M to be not more than 8%.M={(Vmax−Vmin)/(Vmax+Vmin)}×100  (1)

For the magnetic disks from Comparative Example 1, the modulation M hada large value of 10% or more, whereas for the magnetic disks fromExample 1, the modulation M was low at 4%, showing that the magneticdisks from Example 1 had a good flatness.

INDUSTRIAL APPLICABILITY

According to the substrate for information recording media andmanufacturing method thereof of the present invention, the substrate forinformation recording media is manufactured through only precisionpolishing using ultra-fine abrasive grains of a predetermined graindiameter, or only pre-polishing using fine abrasive grains of apredetermined grain diameter and precision polishing using ultra-fineabrasive grains of a predetermined grain diameter, and without carryingout rough polishing. As a result, by using a starting material glassplate having excellent surface waviness properties, a substrate forinformation recording media having good planarity with excellent surfacewaviness properties can be obtained in a short time and with a lowpolishing amount, which is useful in terms of improving productivity.Moreover, because the substrate for information recording media has avery excellent planarity, it can be ensured that even a small magnetichead follows the substrate well, and hence the substrate for informationrecording media of the present invention can be used as a substrate fortoday's small-sized large-capacity high-recording-density informationrecording media.

Moreover, the starting material glass plate according to the presentinvention has excellent flatness and planarity, and hence can be used invarious applications where a high degree of flatness and planarity isrequired.

1. A method of manufacturing a substrate for information recordingmedia, comprising: carrying out surface polishing on at least onesurface of a sheet-shaped starting material glass plate manufactured bya floating process; wherein a plurality of types of surface wavinessclassified by wavelength band are formed in a fashion superimposed onone another on the at least one surface of the starting material glassplate, and a long-wavelength waviness for which the wavelength band ishighest out of the tyes of surface waviness is formed to not more than 6nm; wherein said surface polishing consists essentially of precisionpolishing using fine abrasive grains having a predetermined graindiameter.
 2. A method of manufacturing a substrate for informationrecording media, comprising: carrying out surface polishing on at leastone surface of a sheet-shaped starting material glass plate manufacturedby a floating process; wherein a plurality of types of surface wavinessclassified by wavelength band are formed in a fashion superimposed onone another on the at least one surface of the starting material glassplate, and a long-wavelength waviness for which the wavelength band ishighest out of the types of surface waviness is formed to not more than6 nm; and wherein said surface polishing consists essentially ofpre-polishing and precision polishing, wherein said precision polishingis carried out using ultra-fine abrasive grains having a firstpredetermined grain diameter, and said pre-polishing is carried outusing ultra-fine abrasive grains having a second predetermined graindiameter larger than the first predetermined grain diameter.
 3. A methodas claimed in claim 1 , wherein a mean grain diameter of thepredetermined grain diameter is not more than 1.3 μm, and a 90% diameterof a volume grain size distribution thereof is not more than 3.5 μm. 4.A method as claimed in claim 1, wherein a mean grain diameter of thepredetermined grain diameter is not less than 0.01 μm, and a 90%diameter of a volume grain size distribution thereof is not less than0.02 μm.
 5. A method as claimed in claim 2, wherein a mean graindiameter of the second predetermined grain diameter is in a range of 0.3μm to 5 μm, and a 90% diameter of a volume grain size distributionthereof is in a range of 1 μm to 15 μm.
 6. A method as claimed in claim1, wherein the long-wavelength waviness is formed to not less than 0.4nm.
 7. A method as claimed in claim 1, wherein the starting materialglass plate is formed such that a short-wavelength waviness for whichthe wavelength band is lowest is in a range of 0.1 nm to 0.7 nm, and amedium-wavelength waviness for which the wavelength band is intermediatebetween the wavelength band of the long-wavelength waviness and thewavelength band of the short-wavelength waviness is in a range of 0.25nm to 2 nm.
 8. A method as claimed in claim 1, wherein the startingmaterial glass plate is manufactured from a glass ribbon in apredetermined high temperature state formed by pouring a glass rawmaterial onto molten tin.
 9. A method as claimed in claim 1, wherein apolishing amount in said surface polishing is in a range of 1 μm to 75μm from the at least one surface of the starting material glass plate.10. A method as claimed in claim 9, wherein the polishing amount in saidsurface polishing is in a range of ham to 25 μm from the at least onesurface of the starting material glass plate.
 11. A method as claimed inclaim 1, wherein the ultra-fine abrasive grains used in said surfacepolishing contain at least one substance selected from the groupconsisting of cerium oxide, aluminum oxide, zirconium oxide, siliconoxide and manganese oxide.
 12. A method as claimed in claim 11, whereinthe ultra-fine abrasive grains used in said surface polishing comprisecerium oxide.
 13. A method as claimed in claim 2, wherein a mean graindiameter of the first predetermined grain diameter is not more than 1.3μm, and a 90% diameter of a volume grain size distribution thereof isnot more than 3.5 μm.
 14. A method as claimed in claim 2, wherein a meangrain diameter of the first predetermined grain diameter is not lessthan 0.01 μm, and a 90% diameter of a volume grain size distributionthereof is not less than 0.02 μm.
 15. A method as claimed in claim 3,wherein a mean grain diameter of the predetermined grain diameter is notless than 0.01 μm, and a 90% diameter of a volume grain sizedistribution thereof is not less than 0.02 μm.
 16. A method as claimedin claim 13, wherein a mean grain diameter of the first predeterminedgrain diameter is not less than 0.01 μm, and a 90% diameter of a volumegrain size distribution thereof is not less than 0.02 μm.
 17. A methodas claimed in claim 2, wherein the long-wavelength waviness is formed tonot less than 0.4 nm.
 18. A method as claimed in claim 2, wherein thestarting material glass plate is formed such that a short-wavelengthwaviness for which the wavelength band is lowest is in a range of 0.1 nmto 0.7 nm, and a medium-wavelength waviness for which the wavelengthband is intermediate between the wavelength band of the long-wavelengthwaviness and the wavelength band of the short-wavelength waviness is ina range of 0.25 nm to 2 nm.
 19. A method as claimed in claim 6, whereinthe starting material glass plate is formed such that a short-wavelengthwaviness for which the wavelength band is lowest is in a range of 0.1 nmto 0.6 nm, and a medium-wavelength waviness for which the wavelengthband is intermediate between the wavelength band of the long-wavelengthwaviness and the wavelength band of the short-wavelength waviness is ina range of 0.25 nm to 2 nm.
 20. A method as claimed in claim 2, whereinthe starting material glass plate is manufactured from a glass ribbon ina predetermined high temperature state formed by pouring a glass rawmaterial onto molten tin.
 21. A method as claimed in claim 2, wherein apolishing amount in said surface polishing is in a range of 1 μm to 75μm from the at least one surface of the starting material glass plate.22. A method as claimed in claim 21, wherein the polishing amount insaid surface polishing is in a range of 1 μm to 25 μm from the at leastone surface of the starting material glass plate.
 23. A method asclaimed in claim 2, wherein the ultra-fine abrasive grains used in saidsurface polishing contain at least one substance selected from the groupconsisting of cerium oxide, aluminum oxide, zirconium oxide, siliconoxide and manganese oxide.
 24. A method as claimed in claim 23, whereinthe ultra-fine abrasive grains used in said surface polishing comprisecerium oxide.